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Pervious Pavement
Background: impervious cover Impervious cover, defined as hard surfaces that do not allow water through to the soil such as roadways, sidewalks, and patios http://www.coastal.ca.gov/nps/watercyclefacts.pdf 1, is an increasing problem for the water cycle in urban areas. As impervious cover increases in an area, water return to groundwater decreases, and the percent of water that goes to runoff increases. This can lead to more flooding, stream erosion, and less recharge water entering the groundwater. Pervious pavement Pervious pavement is an alternative to traditional concrete, and can help decrease impervious cover. Like traditional concrete, pervious pavement is made from a mixture of cement, water and coarse aggregates http://www.concretenetwork.com/pervious/how_it_works.html 2. However, it does not contain sand, which creates a void content of 15 to 25% within the pavement http://www.perviouspavement.org/ 3. Voids allow water from the top of the pavement to percolate through to the soil beneath, reducing runoff and increasing recharge to groundwater. Typical flow rates through pervious pavement are between 3 and 8 gallons per minute http://www.concretethinker.com/Papers.aspx?DocId=10 4, http://www.perviouspavement.org/performance/video.html 5. Problems associated with pervious pavement While pervious pavement can achieve compressive and flexural strengths comparable to that of traditional concrete, there are several limitations which must be taken in to account. First, due to the void spaces present in pervious pavement, some amount of surface raveling is common http://www.concretenetwork.com/pervious/ 6. Therefore, it is not recommended for use in high-traffic or high-speed roadways http://www.cabmphandbooks.com/Documents/Development/SD-20.pdf 7. Additionally, a vacuuming technique has been developed to help clear the voids of excess material http://www.dot.state.mn.us/mnroad/reports/PDF%27s/porousvacuuming.pdf 8, http://clear.uconn.edu/projects/tmdl/library/papers/dietz_2007.pdf 9. There have also been concerns about the effectiveness of pervious pavement in cold weather. While one summary http://clear.uconn.edu/projects/tmdl/library/papers/dietz_2007.pdf 9 concluded that water movement through the pavement is not altered in cold weather, many sources caution that pervious pavement may not remain effective at removing pollutants after exposure to subzero weather http://www.epa.gov/npdes/pubs/porouspa.pdf 10. Due to the high flow rates that pervious pavement achieves, contamination of the groundwater is a major concern. This is especially true in areas with high impervious cover, since water could run off the impervious pavement, collecting any pollutants present, and drain through the pervious pavement. Therefore, care must be taken to create a suitable surface underneath the pervious pavement which is able to temporarily store the water coming through, such as a gravel bed and filter cloth http://www.epa.gov/npdes/pubs/porouspa.pdf 10. The increased retention time of the water in the gravel allows aerobic bacteria to break down harmful pollutants http://www.concretenetwork.com/pervious/how_it_works.html 2 The future of pervious pavement Pervious pavement has been used in a number of projects, including parking lots in Arizona and a Park and Ride in Glendale http://www.perviouspavement.org/performance/video.html 5. Many people feel that if proper care is taken, pervious pavement can be an asset in working towards a more sustainable water cycle in urban areas http://www.concretethinker.com/Papers.aspx?DocId=10 4. As a step in that direction, the National Ready Mixed Concrete Association has developed a certification program designed to ensure that pervious pavement is prepared and placed correctly http://nrmca.org/Education/Certifications/Pervious_Contractor.htm 11. Index 1: http://www.coastal.ca.gov/nps/watercyclefacts.pdf 2: http://www.concretenetwork.com/pervious/how_it_works.html 3: http://www.perviouspavement.org/ 4: http://www.concretethinker.com/Papers.aspx?DocId=10 5: http://www.perviouspavement.org/performance/video.html 6: http://www.concretenetwork.com/pervious/ 7: http://www.cabmphandbooks.com/Documents/Development/SD-20.pdf 8: http://www.dot.state.mn.us/mnroad/reports/PDF%27s/porousvacuuming.pdf 9: http://clear.uconn.edu/projects/tmdl/library/papers/dietz_2007.pdf 10: http://www.epa.gov/npdes/pubs/porouspa.pdf 11: http://nrmca.org/Education/Certifications/Pervious_Contractor.htm